RE: [net-next 1/2] dummy: add device MTU validation check

-----Original Message-----
From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
Sent: 2017年9月21日 23:02
To: Zhang Shengju <redacted>
Cc: davem@davemloft.net; willemb@google.com;
stephen@networkplumber.org; netdev@vger.kernel.org
Subject: Re: [net-next 1/2] dummy: add device MTU validation check

On Thu, 2017-09-21 at 21:32 +0800, Zhang Shengju wrote:

quoted

Currently, any mtu value can be assigned when adding a new dummy device:
[~]# ip link add name dummy1 mtu 100000 type dummy [~]# ip link show
dummy1
15: dummy1: <BROADCAST,NOARP> mtu 100000 qdisc noop state DOWN

mode DEFAULT group default qlen 1000

quoted

    link/ether 0a:61:6b:16:14:ce brd ff:ff:ff:ff:ff:ff

This patch adds device MTU validation check.

What is wrong with big MTU on dummy ?

If this is a generic rule, this check should belong in core network stack.

dummy_setup() function setup mtu range: [0, ETH_MAX_MTU]. 
This will be checked at dev_set_mtu() function in core network stack.

So if you add a new dummy device without specify mtu value, you can't set a value out of range [0, ETH_MAX_MTU] afterward.
BUT you can set any mtu when adding new device. This cause an inconsistence.

quoted

Signed-off-by: Zhang Shengju <redacted>
---
 drivers/net/dummy.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/dummy.c b/drivers/net/dummy.c index

e31ab3b..0276b2b 100644

--- a/drivers/net/dummy.c
+++ b/drivers/net/dummy.c

@@ -365,6 +365,14 @@ static int dummy_validate(struct nlattr *tb[], struct

nlattr *data[],

quoted

 		if (!is_valid_ether_addr(nla_data(tb[IFLA_ADDRESS])))
 			return -EADDRNOTAVAIL;
 	}
+
+	if (tb[IFLA_MTU]) {
+		u32 mtu = nla_get_u32(tb[IFLA_MTU]);

You do not verify/validate nla_len(tb[IFLA_MTU]).

Do not ever trust user space.

MTU attribute is just u32, do you think it's necessary to check the length? 
Actually I don't see any place to check the length of mtu attribute in network stack code. 

quoted

+
+		if (mtu > ETH_MAX_MTU)
+			return -EINVAL;
+	}
+
 	return 0;
 }